1. Field of the Invention
This invention relates to a process for controlling a bonding gas system and more specifically for controlling the temperature and composition of an activating gas for bonding nonwoven webs of nylon filaments.
2. Description of the Prior Art
Nonwoven webs comprised of a plurality of substantially continuously and randomly deposited filaments of a thermoplastic polymer are well known. There are many different processes for preparing nonwoven webs and, as initially prepared, such webs lack adequate strength and other desirable physical properties necessary for commercial utility. It is therefore conventional practice to strengthen the webs by bonding the filaments together.
Processes for forming and bonding nonwoven webs in a continuous manner are disclosed in U.S. Pat. No. 3,542,615 for "Process for Producing a Nylon Nonwoven Fabric," U.S. Pat. No. 3,676,244 for "Process for Forming High Strength Spun Bonded Fabric by Autogneous Bonding of Filaments," and U.S. Pat. No. 3,705,068 for "Process and Apparatus for Producing Nonwoven Fabrics." In these patents bonding is accomplished by advancing a web through a chamber filled with an activating gas wherein residence time is sufficient to permit absorption of gas into the filaments. The preferred activating gas is hydrogen chloride. These patents disclose preparing a web of nonwoven continuous filaments spun from molten polyamide, pneumatically attenuating the filaments and then depositing them on a conveyor belt to form a coherent, uniform web. The web is then passed into a chamber where it is subjected to a hydrogen chloride atmosphere. Bonding occurs autogeneously at the filament cross-over points. After sufficient residence time in the chamber or gas box to permit surface absorption of the activating gas the web is then compacted, washed and collected.
For an efficient and economical process, control of the activating gas entering the gas box is absolutely essential. To insure adequate bonding at the filament cross-over points, the concentration and temperature of the gas must be carefully controlled and monitored. An economical process is one that is characterized by a high product yield. High yields are attained by producing a uniform product having minimal defects. Satisfactory control of the activating gas is therefore critical to an economical, high yield process. For example, the surface quality of nonwoven webs can be damaged thus lowering yield by the activating gas when the acid concentration is not maintained within carefully prescribed limits.
The activating gas employed in this bonding system consists of three components, air, HCl and water. Bonding is accomplished by contacting the web with a mixture of hydrogen chloride, water and air for a time sufficient to allow HCl and water to be absorbed by the nylon filaments. The rate of absorption of HCl and water is dependent upon several parameters including the initial water content of the filaments, the HCl and H.sub.2 O concentrations in the bonding gas and the bonding gas temperature. For this reason, it is necessary to have precise control over the gas concentration and temperature in order to regulate the degree of bonding achieved. Two systems have been used in the processes disclosed in the heretofore referenced patents for supplying the activating gas. Each system has had varying degrees of success and each is characterized by operating disadvantages peculiar to that specific system.
One system is a single pass one that furnishes an air stream of sufficient volume to provide the required air flow. The air stream is conditioned through an air conditioning train to the desired temperature and moisture content. Gaseous HCl is then metered into this conditioned air stream thereby providing the desired acid concentration. The gas is passed once through the gas box. After contacting the web, the exiting stream is scrubbed with water to remove the unconsumed HCl, demisted and discharged into the atmosphere.
A second system is a recirculating one wherein the activating gas consists of two-thirds recycled gas and one-third make-up air. A portion of the HCl-water-air mixture that was discharged from the gas box is recycled and the unused portion is scrubbed with water, demisted and discarded. The balance of the activating gas is provided by passing an air stream through a conditioning train and then mixing it with the recycled portion. All of the gas cannot be recycled because constant gas temperature necessitates removing the heat of compression supplied by a recirculating blower. This is accomplished by regulating make-up air temperature below the return gas temperature so that the resulting gas mixture is maintained at the correct temperature. It is not possible to use a normal heat exchanger to remove the heat of compression because the HCl and water tend to condense.
Both of these systems are characterized by the disadvantages of (1) discarding relatively large quantities of HCl, this is both expensive and it also poses an ecology problem, and (2) requiring a precisely controlled, low moisture content make-up air stream. Furthermore, the recirculating system is inherently unstable because ambient air containing an uncontrolled amount of water at a fluctuating temperature is constantly being drawn into the gas box during recycling.